The Triplet Code (AQA A Level Biology): Revision Note

Exam code: 7402

Lára Marie McIvor

Written by: Lára Marie McIvor

Reviewed by: Cara Head

Updated on

The triplet code

  • The DNA nucleotide base code found within a gene is a three-letter, or triplet, code

  • Each triplet codes for one amino acid

  • There are four bases (A, G, C, T), so there are 64 different triplets possible (43), yet there are only 20 amino acids that commonly occur in biological proteins

    • This results in multiple triplets coding for the same amino acids; thus, the code is said to be degenerate (this can limit the effect of mutations)

  • Some triplets code for start (TAC – methionine) and stop signals

    • These signals tell the cell where individual genes start and stop

    • This ensures the cell

      • reads the DNA correctly as the code is non-overlapping

      • can produce the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly

  • The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)

  • This means that the same triplet codes for the same amino acids are found in all living organisms

Diagram illustrating DNA structure, including triplet codes for valine and methionine, with non-transcribed and transcribed strands labelled.
A DNA molecule with the triplet code for the codons of the start amino acid (methionine) and valine

Examiner Tips and Tricks

Be careful to determine whether you have been given DNA triplets or mRNA codons to examine in a question. Remembering that the mRNA codon will have been transcribed from the DNA triplet, and so it will have a complementary base sequence to the original DNA sequence.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology, Psychology & Sociology Subject Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Cara Head

Reviewer: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding